Primary reference fuel (PRF) grade heptane contains a minimum of 99.75 weight percent normal heptane (or “n-heptane”) and is generally produced by purifying a heptane stream that contains less than 99.75 weight percent n-heptane. Among the heptane streams available as a feed for production of PRF heptane may be a pure grade heptane, which contains a minimum of 99.0 weight percent n-heptane. Impurities common to heptane streams include the close boiling C7 isomers methylcyclohexane and cis-1,2 dimethylcyclopentane among other C7 olefin, C7 paraffin, and cyclic impurities. Heptane streams, including pure grade heptane and PRF heptane, are valuable commercially as fuels. Additionally, PRF heptane may be valuable as a high purity solvent, such as in pharmaceutical applications. In order to obtain the purity levels associated with PRF heptane, fractionation has been used to separate the normal heptane (n-heptane) from impurities such as other C7 isomers.
Typically, one or more fractionations of a heptane feedstock stream are employed to produce PRF heptane. The high purity of PRF heptane may cause it to require more than one fractionating step. In addition, super-atmospheric fractionation may improve the ability to separate impurities. As a result, the rate of production of PRF heptane may only be a portion of the production rate of heptane grades containing higher levels of contaminants, such as, for example, pure grade heptane. Thus, fractionation of a heptane stream in order to obtain PRF heptane may be both capital intensive and expensive to operate.
The octane number of a fuel is an expression of antiknock properties commonly measured according to ASTM D 2699, ASTM D 2700, or an average of the two. The octane of pure n-heptane is zero, but olefinic and cyclic impurities in a heptane product have significantly higher octane numbers, which raise the octane number of the heptane product such as PRF heptane. Thus, the higher the percentage of impurities removed from a heptane stream, even after attaining the 99.75 weight percent specification for PRF heptane, the closer the octane number is to zero, and the more competitive the product. Thus, a need exists for improved methods of purifying heptane.